1. Field of the Invention
The present invention relates to aircraft communication radios; and more particularly to data link radios for communicating data with different ground stations.
2. Description of the Related Art
Aircraft commonly have a very high frequency (VHF) radio that provides a bidirectional data link to support air traffic control and airline operational control. This “data link” radio can be used to transmit and receive tactical data, and navigational messages between the aircraft and ground stations. The data link radio is part of the Aircraft Communications Addressing and Reporting System (ACARS) that comprises networks of ground stations covering most of the developed areas of the world that provide communications for airline operational control, air traffic services, and air traffic control.
In the original implementation of ACARS, ground stations operated by different service providers and in different geographical regions were assigned different frequencies (VHF channels). Those ACARS radios employ a communication protocol referred to as Mode A in which the data is transmitted at 2400 bits per second using minimum shift keying (MSK) modulation that produces a signal which modulates the VHF carrier. The result is a radio frequency signal that is double side-band amplitude modulated at 2400 bits per second (DSB AM-MSK).
As the use of data link radios grew, the ground stations geographical areas with large amounts of aircraft traffic began operating at or near full capacity and it was not practical to provide more radio frequencies for data link use. In order to accommodate an expanding need for greater communication capabilities, ACARS was enhanced with another data link protocol, referred to as Mode 2. Initially all Mode 2 ground stations operated on the same radio frequency, however, recently some Mode 2 ground stations have been assigned to secondary frequencies. This protocol employs a digital modulation scheme and a significantly higher data rate of 31,500 bits per second, thereby making more efficient use of the VHF channel than the Mode A protocol. The increase in data transfer capacity is made possible by utilizing the differential eight phase shift keying (D8PSK) modulation technique. This allows the same number of VHF ground stations that previously operated with the Mode A protocol to serve a greater number of aircraft by switching to the Mode 2 protocol. Because of that very high data transfer rate, many the Mode 2 stations are able to share the same frequency by utilizing signal collision avoidance algorithms.
Regardless of the protocol, in order to establish communication with a particular ground station, the radio receiver onboard an aircraft first listens for a station information message on the VHF channel. The station information message, sometimes called a “squitter”, is transmitted periodically, e.g. once every two minutes, to inform aircraft radios that the particular ground station is active and generally available for communication. Upon receiving a station information message, the aircraft data link radio system establishes data communication.
As an aircraft travels, it moves beyond the range of one ground station and needs to establish communication with a new ground station closer to the aircraft's present position. If the a station is operating in a different mode, heretofore that required that the aircraft data link radio system to wait up to two minutes for a station information message or a permission to send message from a new station before communication could commence. Furthermore, the radio system may have to be configured manually for that different communication mode. Also even though some areas of heavy air traffic, such as the Northeast region of the United States of America, are serviced by both Mode 2 and Mode A stations, an aircraft approaching that area using the Mode A protocol may have to switch to Mode 2 protocol because the Mode A stations ahead are operating at maximum capacity. In each of these situations, the data radio onboard the aircraft had to wait until a station information message or a permission to send message was received from a new station.
It is desirable to automate the transfer of aircraft communication from one ground station to another with as minimal delay time and with minimal manual intervention by the flight crew.